Ethanol for industrial use is conventionally produced from petrochemical feed stocks, such as oil, natural gas, or coal; from feed stock intermediates, such as syngas; or from starchy materials or cellulose materials, such as corn and sugar cane. Conventional methods for producing ethanol from petrochemical feed stocks, as well as from cellulose materials, include the acid-catalyzed hydration of ethylene, methanol homologation, direct alcohol synthesis, and Fischer-Tropsch synthesis. Instability in petrochemical feed stock prices contributes to fluctuations in the cost of conventionally produced ethanol, making the need for alternative sources of ethanol production all the greater when feed stock prices rise. Starchy materials, as well as cellulose material, are often converted to ethanol by fermentation. However, fermentation is typically used for consumer production of ethanol. In addition, fermentation of starchy or cellulose materials competes with food sources and places restraints on the amount of ethanol that can be produced for industrial use.
Ethanol production via the reduction of alkanoic acids and/or other carbonyl group-containing compounds has been widely studied, and a variety of combinations of catalysts, supports, and operating conditions have been mentioned in the literature. In the reduction of an alkanoic acid, such as acetic acid, water may be formed in an equal molar ratio with ethanol.
Several processes that produce ethanol from acetic acid, and esters, including methyl acetate and ethyl acetate, are described in the literature.
EP02060553 describes a process for converting hydrocarbons to ethanol involving converting the hydrocarbons to ethanoic acid and hydrogenating the ethanoic acid to ethanol. The stream from the hydrogenation reactor is separated to obtain an ethanol stream and a stream of acetic acid and ethyl acetate, which is recycled to the hydrogenation reactor.
WO2009063174 describes a continuous process for the production of ethanol from a carbonaceous feedstock. The carbonaceous feedstock is first converted to synthesis gas which is then converted to ethanoic acid, which is then esterified and which is then hydrogenated to produce ethanol.
WO2009009320 describes an indirect route for producing ethanol. Carbohydrates are fermented under homoacidogenic conditions to form acetic acid. The acetic acid is esterified with a primary alcohol having at least 4 carbon atoms and hydrogenating the ester to form ethanol.
US Pub. No. 20110046421 describes a process for producing ethanol comprising converting carbonaceous feedstock to syngas and converting the syngas to methanol. Methanol is carbonylated to ethanoic acid, which is then subjected to a two stage hydrogenation process. First the ethanoic acid is converted to ethyl ethanoate followed by a secondary hydrogenation to ethanol.
U.S. Pat. No. 7,884,253 describes a process for producing ethanol by converting syngas to methanol and catalytically converting the methanol into acetic acid. The acetic acid along with methanol is esterified to generate an acetate. The acetate is reduced with hydrogen to produce ethanol.
A widely used and successful commercial process for synthesizing acetic acid involves the catalyzed carbonylation of methanol with carbon monoxide. The catalysis contains rhodium and/or iridium and a halogen promoter, typically methyl iodide. The reaction is conducted by continuously bubbling carbon monoxide through a liquid reaction medium in which the catalyst is dissolved. The reaction medium also comprises methyl acetate, water, methyl iodide and the catalyst. Conventional commercial processes for carbonylation of methanol include those described in U.S. Pat. Nos. 3,769,329, 5,001,259, 5,026,908, and 5,144,068, the entire contents and disclosures of which are hereby incorporated by reference. Another conventional methanol carbonylation process includes the Cativa™ process, which is discussed in Jones, J. H. (2002), The Cativa™ Process for the Manufacture of Acetic Acid, Platinum Metals Review, 44 (3): 94-105, the entire content and disclosure of which is hereby incorporated by reference.
The crude acetic acid product from the reactor is processed in a purification section to remove impurities and recover acetic acid. These impurities, which may be present in trace amounts, affect the quality of acetic acid, especially as the impurities are circulated through the reaction process, which, among other things, can result in the buildup of these impurities over time. Conventional purification techniques to remove these impurities include treating the acetic acid product streams with oxidizers, ozone, water, methanol, activated-carbon, amines, and the like. The treatments may also be combined with the distillation of the crude acetic acid product. However, the additional treatment of the final product adds cost to the process, and distillation of the treated acetic acid product can result in additional impurities being formed.
Processes for removing these impurities may also remove compounds in the reaction medium, such as the halogen promoter. Several processes have been taught for recovering the halogen promoter including treatment of vented streams and extraction.
Treatment of vented streams allows recovery of halogen promoters. For example, U.S. Publication No. 2009/0270651 discloses a methanol carbonylation system that includes an absorber tower adapted for receiving a vent gas stream and removing methyl iodide therefrom with a scrubber solvent, the absorber tower being coupled to first and second scrubber solvent sources which are capable of supplying different first and second scrubber solvents. A switching system including valves alternatively provides first or second scrubber solvents to the absorber tower and returns the used solvent and absorbed material to the carbonylation system to accommodate different operating modes.
Extraction may also recover halogen promoters from the carbonylation products. For example, U.S. Pat. No. 4,908,477 discloses separating organic iodine compounds from carbonylation products of methanol, methyl acetate and dimethyl ether and from mixtures of such carbonylation products by a process wherein the iodine compounds are removed by liquid phase extraction with a non-aromatic hydrocarbon.
While the above-described processes have been successful in reducing and/or removing impurities from the carbonylation system, further improvements can still be made for removing and recovering the halogen promoters, thus allowing a feed stream substantially free of halogen promoter to be fed to a hydrogenation reactor.